Ever since its discovery, the laser has been considered for solving telecommunications problems because of the high coherence and bandwidth of a laser beam and the simplicity of focusing the laser beam. Laser light and optical fibers are becoming increasingly important in the communications industry. Just as the gap between Einstein's theory (1917) and the building of the first laser (1960) was due to lack of equipment, the maximum exploitation of lasers in telecommunication remains theoretical until key technological developments are realized.
Lasers (Light Amplification through Stimulated Emission of Radiation) can produce continuous or pulsed emission. There are two ways to produce pulse emission: gain switch and modelocking. It is well known that gain switch lasers can provide short optical pulses on the order of milliseconds to microseconds. Modelocked lasers can produce shorter optical pulses, on the order of pico seconds or shorter.
Large laser systems based on modelocked YAG or YIF are used to generate short optical pulse for testing the time response of fiber optic communication system. The time response is related to the maximum rate of transmitting information. As the speed of telecommunications systems increase, the need for a sub-picosecond testing impulse source also becomes more acute.
Since becoming commercially available, Erbium doped fiber has become the preferred gain medium for generating short optical pulses in actively and passively modelocked lasers. Thus, technological improvements have contributed to progress toward creating a laser pulse source for testing the time response of telecommunication systems and for generating repetitive pulses at high data rates. However, a laser which can maintain a single polarization state, essential to maintaining optimal laser activity, has remained a technical barrier.
The polarization state of light can be described by the amplitudes and phase relationship of the two oscillating fields of a light wave. In general, output from a laser or fiber laser has a well-defined single polarization state with a high order of degree of polarization. As this output light propagates in an isotropic medium (i.e. a medium with no birefringence--light travels at the same speed along both axes) the polarization state will not change. However, the output polarization state will vary if light does not travel along the birefringent axis of an anisotropic (birefringent) medium. Although single mode fiber is known to have a very small intrinsic birefringence, it is susceptible to external perturbation (e.g. due to bending, temperature change), which results in changes in birefringence and changes in the polarization state result.
Inside a fiber laser cavity, this externally-induced birefringence will cause the exit, or output, polarization state to vary, if the effect is small, or to cease laser activity entirely if the effect is large. An internal adjustable polarization control element is required in order to maintain the fiber laser in optimal laser condition. This requires constant surveillance and adjustment. The need exists for a polarization independent fiber laser which does not require constant adjusting of the polarization state in the single mode fiber.
Attempts to obtain a well-defined pulse which does not require ongoing adjustment of polarization have been unsuccessful. Currently, polarization controllers are needed to adjust polarization states inside a fiber laser cavity in single frequency ring lasers, and actively and passively modelocked picosecond fiber lasers.
Polarization adjustment is needed because changes in the external environment create variation in polarization birefringence which, in turn, causes variation in polarization states. Variation in polarization states due to changes in birefringence in a single mode fiber means that the amplitude and phase relationship are changed according to the polarization state variation.
Configurations which eliminate optical birefringence effects on the polarization state of a single pass beam have been reported. See Martinelli, "A Universal Compensator for Polarization Changes Induced By Birefringence on a Retracing Beam", Optics Communications, vol 72, number 6, pp 341-344 (1989). This device, depicted in FIG. 1, is operated based on the symmetries induced by a 45 degree Faraday rotator 12 followed by a mirror 14. The entrance and exit polarization states of the input and output beams 16, 18 turn out to be orthogonal and independent of the arbitrary birefringence material 20. The arbitrary birefringence material 20 changes the input beams 12 known polarization state into an arbitrary elliptical unknown polarization state 22.
The 45 degree Faraday rotator rotates the major axes of the arbitrary elliptically polarized light beam 22 by 45 degrees without changing its ellipticity and handedness (right or left handed) to a new polarization state 24. The mirror 14 changes the handedness of the reflected beam 26 from right to left (or left to right, depending on the initial state) without affecting the ellipticity and the orientation of the major axes of the elliptically polarized beam. The 45 degree Faraday rotator rotates the major axis of the elliptically polarized reflected beam 26 another 45 degrees without changing its handedness (left or right handed) and ellipticity to the output beams 18 orthogonally polarized state 28 which is orthogonal to that of the input beam's 16 arbitrary elliptically polarized state.
As the orthogonally polarized reflected beam 28 passes the arbitrary birefringence material 20, the beam sees opposed birefringence when compared to the input beam 16 and cancels the birefringence induced to the input beam 16. Cancellation of birefringence has the practical effect of switching the fast and slow axis of the arbitrary birefringence material 20 and undoing the birefringence effect induced to the input beam 16 by the arbitrary birefringence material 20. As a result, the output or exit polarization 18 will still be well-defined, but it will be orthogonal to that of the known input or entrance beam 16 polarization state.
In sum, this means:
a) for linearly polarized input beam, the output polarization state will remain linearly polarized with its polarization axes rotated by 90 degrees; PA1 b) for circularly polarized input beam (right or left handed), the output polarization state will remain circularly polarized with opposite handedness (left or right handed); PA1 c) for elliptically polarized input beam, the output polarization state will remain elliptically polarized with its major axis rotated 90 degrees, opposite handedness and unchanged ellipticity.
Without the 45 degree Faraday rotator, the relationship of the output polarization state 18 (after passing the arbitrary material 20, reflected by mirror 14, and passing the arbitrary medium the second time) to the input polarization will be random, not well-defined, and will vary with changes in the external environment. Thus, the addition of the 45 degree Faraday rotator created a persistent, well-defined relationship between the input and output polarization states.
This principle of using a Faraday rotator to cancel birefringence is used by Duling and Esman in their recent paper describing a linear input followed by a 45 degree Faraday rotator and a mirror. The resulting output is orthogonal to the input and the amplitude of the output is increased. See Duling and Esman, "A Single Doped Er-Doped Fiber Amplifier", 1992 Conference of Lasers and Electro-optics, CPD 28-1/60, Duling and Esman (1992).
As depicted in FIG. 2, Duling et al. report a single polarization fiber amplifier which consists of a polarization beam splitter 30, a standard (commercially available) Erbium-doped fiber 32, and a Faraday rotator mirror 34. On retracing its path, light is everywhere orthogonal to the first pass light and is linear when reflected. Horizontally linearly polarized light from a polarization maintaining (PM) fiber is input from port 1 through the polarization beam splitter 30 and into the nonpolarization maintaining Erbium-doped fiber 32. The light is then reflected back from the Faraday rotator 34 for a second pass through the Erbium-doped fiber 32. On retracing its path, the polarization state of the second pass light is everywhere orthogonal to the polarization state of the first pass light and is vertically linearly polarized as it exits through the polarization beam splitter 30 out to port 2.
Duling has also described two possible laser configurations based on this single polarization fiber amplifier. FIG. 3 shows a narrow linewidth using an additional mirror 36 and a grating 38 to produce continuous wave (CW) laser action that is independent of the birefringence of the fiber. In this laser cavity, two reflective ports are required together with the Faraday rotator 34 to sustain the laser action.
Duling also demonstrated the use of the single frequency, single polarization fiber traveling wave amplifier together with polarization maintaining fiber 40 to build a narrow linewidth in-line fiber laser. (See FIG. 4).
In summary, Duling demonstrated two CW lasers based on use of single polarization fiber amplifier. Thus, a single Faraday rotator has been used to cancel birefringence in a fiber laser. However, there still remains a need for an ultrashort pulse fiber laser which does not require constant tuning and adjusting to control polarization states and thereby optimize laser action.